PGM sulfide concentrates have a sulfur content of about 2 to 7 wt-%. This sulfur content is too high to allow treatment of the concentrates directly in an electric furnace or the like by separating SO2, but also too low to process the concentrates in a conventional overall plant comprising roaster, gas cleaning and sulfuric acid plant, since the concentration of the SO2 gas generated would be too low to be processed in an autothermally operated sulfuric acid plant.
EP 1 157 139 B1 describes a process in which the concentrate is roasted completely and subsequently smelted in an electric arc furnace under reducing conditions, so that metal or metals are accumulated in an alloy. The roasting operation is performed in a fluidized-bed reactor such that a constant flow of SO2-containing gas is formed, which can be used for charging a sulfuric acid production. The SO2-containing gas released can be subjected to a gas cleaning. There is not shown a way how to achieve a rather low sulfur content of the roasted concentrate.
The publication “ConRoast: DC arc smelting of dead roasted sulphide concentrates” (http://www.pyrometallurgie.co.za./MINTEK/ConRoast.ConRoast.html) describes a process for the treatment of nickel, copper and PGM sulfide concentrate, in which the sulfur is withdrawn from the roasting reactor and used in a continuous stream of SO2-containing waste gas for charging a sulfuric acid plant. The dead roasted concentrate is molten in a DC electric arc furnace by using an iron-based alloy as collector for nickel, copper, cobalt and PGMs.